Draggable: make revert a method

[ivan]

A pretty common need with draggables/droppables is to accept the drop, do some processing, and then, if some conditions are not met, restore the draggable to the original position.
I think it would make sense to make 'revert' callable externally (for example in the 'drop' method of the droppable).
The attached patch does that: unfortunately, at the moment it requires setting draggableInstance.cancelHelperRemoval = true; in the draggable 'stop' or in the droppable 'drop' methods, to prevent the helper from being removed too soon (see also here for a discussion).